diff -r dfacdb971494 -r 881c5fbeb849 src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp	Thu Jan 31 01:17:41 2019 +0100
+++ b/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp	Thu Jan 31 10:31:39 2019 +0100
@@ -65,6 +65,8 @@
 #include "gc/shenandoah/heuristics/shenandoahTraversalHeuristics.hpp"
 
 #include "memory/metaspace.hpp"
+#include "runtime/interfaceSupport.inline.hpp"
+#include "runtime/safepointMechanism.hpp"
 #include "runtime/vmThread.hpp"
 #include "services/mallocTracker.hpp"
 
@@ -1863,6 +1865,22 @@
   return _free_set->used();
 }
 
+bool ShenandoahHeap::try_cancel_gc() {
+  while (true) {
+    jbyte prev = _cancelled_gc.cmpxchg(CANCELLED, CANCELLABLE);
+    if (prev == CANCELLABLE) return true;
+    else if (prev == CANCELLED) return false;
+    assert(ShenandoahSuspendibleWorkers, "should not get here when not using suspendible workers");
+    assert(prev == NOT_CANCELLED, "must be NOT_CANCELLED");
+    {
+      // We need to provide a safepoint here, otherwise we might
+      // spin forever if a SP is pending.
+      ThreadBlockInVM sp(JavaThread::current());
+      SpinPause();
+    }
+  }
+}
+
 void ShenandoahHeap::cancel_gc(GCCause::Cause cause) {
   if (try_cancel_gc()) {
     FormatBuffer<> msg("Cancelling GC: %s", GCCause::to_string(cause));